1. Field of the Invention
The invention relates to a color image processing apparatus and, more particularly, to color drift correction in a color image.
2. Description of the Related Art
In a color printer as a color image processing apparatus, a plurality of printing mechanisms for printing images of different colors are arranged and printing is executed by using toner of cyan, magenta, yellow, and black. In such a color printer, since the images recorded by the toner of four colors are overlaid onto a record medium every color, a color drift is liable to occur. Ordinarily, the color drift of the printer occurs in the vertical or lateral direction of the record medium or in a state where the color drifts are combined in both of the vertical and lateral directions. Since the color drift occurs by various causes such as error of an attaching position of an LED head, variation in the image record medium, variation in conveying speed of the image record medium, rotational variation due to an eccentricity of a rotor, and the like, it is difficult to perfectly eliminate the color drift. Therefore, the color drifts which remain because they could not be eliminated are generally corrected by an image process called a trapping process. An outline of the trapping process will now be described with reference to the diagrams.
FIGS. 21A, 21B, and 21C are explanatory diagrams of the trapping process.
FIG. 21A is the diagram showing a state where a white-on-black portion has occurred due to the color drift. FIG. 21B is the diagram showing a state where a background area has been expanded by the trapping process. FIG. 21C is the diagram showing an area where the background expanded by the trapping process and an object have been overlaid. For example, it is assumed that the color drift of a dimension α occurred on the left side of an object (magenta) as shown in FIG. 21A. In such a case, to correct the color drift of the dimension α, the area of the object (magenta) is unconditionally expanded in the whole circumference by (α+Δα) as shown in FIG. 21B. (Refer to JP-A-2000-106628).
According to the conventional trapping process, the following problem to be solved remains. That is, even if the color drift of the printer occurred only in the lateral direction, by executing the trapping process, the overlapped area is also extended in the vertical direction where the correction is inherently unnecessary or extended to a portion where the overlap of the color images has already inherently existed due to the color drift.